Determination of the rate of synthesis of cerebral serotonin until now has involved tissue sampling and measurement of the concentration of various tryptophan metabolites in animals that have been treated with a specific enzyme-inhabiting drugs. Because the specific inhibitory action of the drug alters the steady-state balance of the amine, other processes, such as feedback regulation, may be called into play; as a result, the rates estimated from such experiments may not reflect the true rate of synthesis of serotonin in the brain. This method makes use of 14C-labeled alpha-methyl-L-tryptophan, which is converted in the brain to labelled alpha-methylserotonin (alpha-M5HT), a metabolite that persists in the brain for long times and will be applied to an autoradiographic study of rats given drugs known to influence the synthesis of serotonin. In an initial experiment rats will receive reserpine, in order to evaluate for the first time the influence of this drug on (a) the regional synthesis rates, and (b) the anterograde rate of transport of serotonin and/or 5-hydroxytryptophan (there is probably little of 5-hydroxy tryptophan present in these neurons) in fibres of the medial forebrain bundle. The influence of inhibitors of MAO (e.g. pargyline). AAAD (e.g. NSD-1015), and of the 5HIAA-transport system (probenecid), all of which have been used in previous methods of assessment of the rate of serotonin synthesis in rat brain, will be evaluated in detail by this autoradiographic method. The effect of lesions in the hypothalamus and raphe nuclei on the rate of transport of serotonin from the dorsal and medial raphe nuclei will be studied. In addition to measurements of the 5HT axonal transport, we will assess the rate of transport of proteins after local injection of [3H] proline. Additional measurements of the rate of serotonin synthesis will be done for dog and human brain; these will employ 11C-labelled alpha-methyl-L- tryptophan in association with positron-emission tomography (PET). In dog we will evaluate the effect of reserpine, and of inhibitors of AAAD and MAO, on the rate of serotonin synthesis, using each animal in the control and treated states. During the third year we expect to accomplish some control measurements of the regional rate of serotonin formation in human subjects, and will continue with studies of depressed patients (not requested in this application), and the effect of their clinical treatment on the synthesis of serotonin.